While he was settling into his new life in the United States in 1991 Simon Deng saw a newspaper headline proclaiming that human beings were for sale in Sudan for 10 dollars.

“It brought out everything I had been through as a child,” he said over coffee at a Jerusalem hotel this week. “I almost lost it. I did not sleep for three days. My choice was to live in denial or come out of the closet. I realised I had to come out and tell the world that that newspaper was true because I myself was a slave as a child.”

Mr Deng has become one of America’s leading activists against slavery and the mass killings by the Sudanese government in Western and South Sudan. He has walked from the United Nations in New York to Capitol Hill in Washington to protest against inaction over Darfur, and met US President Bush. Now in campaigning in Israel, he has met hundreds of Sudanese refugees.

Mr Deng’s visit comes as Israel hardens its position towards the African refugees crossing from Egypt, with Knesset legislators passing on first reading a bill that would provide prison sentences of five to seven years for those making the crossings and would legalise rapid deportations.

Mr Deng, whose forehead is marked with welts identifying him as a member of the Shilluk tribe, recalled that his family had given him up for dead during his three years of bondage to an Arab from Northern Sudan.

“When I was taken at the age of nine I was forced to do things that a human being should not do, to do things that a child at that age is not capable of. In the north, donkeys are used to carry water from the River Nile, but as a slave that became my job. A human being should have the opportunity to say ‘no’ when he is subject to terror, but there was no choice. I was beaten even when I did nothing wrong, just because someone decided he called me and I didn’t say ‘yes’ loudly enough.”

After three years, he met a fellow Shilluk who helped him to escape.

Mr Deng has a track record of strong support for Israel, including denouncing comparisons of Israeli policies with apartheid. But he is dismayed with what he says is a policy of  differentiating between 600 Muslim refugees from Darfur, who have been granted residency, and some 2,000 mostly Christian refugees from South Sudan, who face possible expulsion. Israel, he says, should grant the southerners a haven until 2011, when a referendum is due on southern independence. “These are all Sudanese. They faced the same things from the same people.”

Admitting preference was given to Darfur refugees, a Foreign Ministry spokesman said: “Other people we consider to be economic refugees.”